Yes, Emily Jane was first and middle names but listed with step father's surname in the 1870 census. It happened often. Glad you found the marriage record for Mary A Mapes to Emily's father and a sibling for her.
7 June 1870; Census Place: Johnson, Maries, Missouri; Roll: M593_791; Page: 429A; Image: 105; Family History Library Film: 552290. Lanes Prairie Post Office
James G Neighbors age 22 born MO
Mary A Neighbors age 25
Emily Jane Neighbors age 6 born MO
Charles H Neighbors 11/12 MO born June 1869
James G Nabers/Neighbors married Mary Ann Crosslin 08 Mar 1868 Phelps Co MO, which is in my original reply, with two different transcriptions.
You are welcome ! Merry Christmas to you and yours.